1. Field of the Invention
The present application relates to a spin torque oscillator that stably oscillates at a high frequency and a magnetic recording head and a magnetic recording device including a spin torque oscillator for microwave assist recording that copes with high magnetic recording density.
2. Background Art
In recent years, spin electronics elements such as a TMR (Tunneling Magneto Resistance) head used in a read head of an HDD (Hard Disk Drive) and a spin injection MRAM (Magnetic Random Access Memory) have substantially contributed to the development of information technology. In a development process of such spin electronics elements, a spin torque oscillator that causes a magnetic body to oscillate using spin torque and generates a microwave, a spin torque diode effect for rectifying a high-frequency current, and the like were found. Possible applications of the spin electronics elements are further expanding to generation, detection, modulation, amplification, and the like of high-frequency waves. In particular, concerning an application of the spin torque oscillator to the HDD, the spin electronics elements attract a great deal of attention as means for attaining high recording density in the future. Concerning problems in an increase in recording density of the HDD and a method of solving the problems and attaining the high recording density, the background is explained more in detail.
According to the improvement of recording density of the HDD, microminiaturization of a bit size of recording media advances year after year. However, as the microminiaturization of the bit size advances, there is a more concern about a loss of a recording state due to thermal fluctuation. In order to solve such a problem and stably maintain a recording bit in high density recording in future, it is necessary to use a recording medium having large coercive force (i.e., large magnetic anisotropy). However, a strong recording magnetic field is necessary in order to perform recording in the recording medium having the large coercive force. However, actually, there is an upper limit in recording magnetic field intensity because of a reduction in width and size of a recording head and a limit in a usable magnetic material. Because of such reasons, the coercive force of the recording medium is restricted by the magnitude of a recording magnetic field that can be generated in the recording head. In order to meet conflicting requests of high thermal stability of a medium and coercive force for easy recording, recording methods of effectively reducing the coercive force of the recording medium only during recording using various assisting means have been devised. Thermal assist recording and the like for performing recording using both a magnetic head and heating means such as a laser are representatives of such recording methods.
On the other hand, there is also an idea for locally reducing the coercive force of the recording medium to perform recording by using a high-frequency magnetic field as a recording magnetic field from the recording head. For example, JP Patent Publication (Kokai) No. 6-243527 A (1994) discloses a technique for Joule-heating or magnetic-resonance-heating a magnetic recording medium with a high-frequency magnetic field and locally reducing medium coercive force to thereby record information. In such a recording method of using magnetic resonance of a high-frequency magnetic field and a magnetic head magnetic field (hereinafter referred to as microwave assist recording), since the magnetic resonance is used, in order to obtain a reduction effect of a reversal magnetic field, it is necessary to apply a large high-frequency magnetic field proportional to an anisotropic magnetic energy of a medium.
In recent years, like the spin torque oscillator, a generation principle for a high-frequency magnetic field using spin torque is proposed and possibility of the microwave assist recording is becoming realistic. For example, in X. Zhu and J. G. Zhu, “Bias-Field-Free Microwave Oscillator Driven by Perpendicularly Polarized Spin Current” IEEE TRANSACTIONS ON MAGNETIC, P2670 VOL. 42, NO. 10 (2006), a calculation result concerning a spin torque oscillator that oscillates without a bias magnetic field from the outside is disclosed. In J. G. Zhu and X. Zhu, ‘Microwave Assisted Magnetic Recording,’ The Magnetic Recording Conference (TMRC) 2007 Paper B6 (2007), a technique for arranging, near a magnetic recording medium adjacent to a main pole of a magnetic head, a magnetization high-speed rotor (Field Generation Layer: FGL), in which magnetization rotates at high speed with spin torque, to generate a microwave (a high-frequency magnetic field) and recording information in a magnetic recording medium having large magnetic anisotropy is disclosed. Further, in J. Zhu and Y. Wang, ‘Microwave Assisted Magnetic Recording with Circular AC Field Generated by Spin Torque Transfer,’ MMM Conference 2008 Paper GA-02 (2008), a spin torque oscillator that controls a circular polarization direction of an FGL using a magnetic field of a main pole adjacent to the FGL is proposed. This makes it possible to efficiently realize microwave assist magnetization reversal of a medium.